Welded metal bellows seals continue to have success as a core sealing technology and have gained popularity recently in new innovative sealing technologies such as high-temperature non-contacting gas lubricated seals and high-temperature corrosion resistant seals. This is especially important in the oil, gas and chemical industries where pumping liquid from one area to another is complicated by great temperature extremes.
Unfortunately, not all seals are created equal. Differing features, materials and a host of other factors can impact the overall effectiveness of the seal. Only with an understanding of these differences can plant operators select the most effective product for the application.
A welded metal bellows seal is made through a process of stamping disc-like plates in specific contoured shapes and welding them in pairs at the inside diameters to form individual convolutions of the bellows. A series of convolutions is then stacked and welded at the outside diameters to form the bellows capsule. Suitable end-fittings complete the assembly.
The welded metal bellows assembly acts as a spring to keep the primary sealing faces together, acts as a dynamic seal and transmits torque from the set screw collar to the seal's rotating face.
Welded bellows have specific advantages, including:
- Higher strength with the ability to withstand greater pressures
- Wider operating temperature range
- Ability to be given precise design characteristics
- Lower spring rate (the amount of force required to compress it a given distance)
- Lower stress in critical areas
- Welded bellows allow for the use of optimal plate shapes such as the Nesting Ripple design
- Static secondary seals
- Only one moving part. . . the bellows
If you've discerned that an edge-welded metal bellows seal is optimal for your application, consider that not all seals are created equal. Differentiating bellows features-the plate shape and thickness, vibration attributes, the impact of double-ply, face angle and more-all impact the effectiveness of the welded metal bellows seal. Operators should understand these differences in order to pick the most effective product for their application as well as extend mean-time between repair, standardize inventory, increase reliability and improve fugitive emission control and water conservation. The following are some key distinctions and features of today's welded metal bellows:
The plate shape influences flexing, stroke and operating length. In the nesting ripple configuration, all plates in the bellows are identical and contoured to permit nesting when compressed. Contouring also improves the bellows' ability to withstand high pressure. The nesting ripple plate shape is more effective in achieving maximum flexing, long (axial motion) stroke with short operating lengths and a low spring rate.
The sweep radius is optimized at 20-25 percent of span and it prevents a phenomenon known as oil-canning-the inversion of the plate geometry that results in a bulging in and out of the plate, similar to that on the bottom of an oil can when it is pressed. Each convolution is made up of a male and female plate, which allows the seal to be designed with a short axial space.
For bellows with straight flat segments, the variability of the microstructure in the heat-affected zones results in less reliable weld joints. By theoretical analysis using linear, thin-shell theory, it has been shown that tilting the bellows axis drastically reduces stresses at the welds and heat-affected zones. The analysis indicates that the stresses at the welds were predominantly bending stresses. With increasing tilt angles, the bending stresses are lowered. This design principle also has been thoroughly documented in both theoretical and empirical studies conducted by an independent government-sponsored agency and verified experimentally. With a 45-deg tilt angle, bending stresses are directed away from the heat-affected zone of the weld. This results in plate rigidity, which adds reliability and reduces fatigue.
State-of-the-art manufacturing processes ensure integrity of the weld by preventing excessive root gap with bead geometry, bead thickness and roll-over control. Bellows units should be checked for leak tight performance with helium mass spectrometry and vacuum-tested to 10-6 TORR. With helium mass spectrometry, the seal is evacuated internally and blanketed in helium. Traces of the gas, which then penetrate through either a break in the weld or a material flaw, are immediately picked up by the sensing probe and the seal is rejected.
Plate Thickness (Thin Plates)
Thin plates provide lower spring rates, which result in lower face loads, less unit loading, less heat generation and longer life than thicker plates. Thicker bellows plates have higher spring rates and are more susceptible to metal fatigue. Repeated plastic deformation of the plates (beyond their plastic limit) during deflection can result in fatigue and greatly reduce seal cycle life.
Increasing the thickness of the plate material, though easier to weld, increases its stiffness and increases the spring rate of the bellows significantly. A high spring rate is undesirable because of the significant changes in loading of the sealing faces with only slight changes in the operating length of the seal. This causes excessive closing force, which in turn causes loss of the lubricating film between the sealing faces, excessive face heat and eventual seal failure. This is especially critical in high temperature and poor lubricating environments.
Bellows with high spring rates are also less capable of compensating for installation problems, shaft movements, impeller adjustments, pump end play, shaft growth due to heat and gradual wearing of the sealing faces. Thinner plates are more difficult to manufacture, which is why many manufacturers are forced to use thicker plates.
Not all bellows in the industry are fitted with vibration dampeners, which help prevent any potential damage from harmonic vibration caused by episodes of dry-running. However, a vibration dampener is ideal. In certain seal designs, the vibration damper pad is a built-in design feature which allows protection-if and when it's needed-against vibrations of a potentially damaging nature.
Double-ply bellows are typically utilized in higher pressure applications and often used in services in which the fluid is thermo-sensitive or has a tendency to set-up and solidify on the seal faces where more start-up torque strength may be required. The double-ply design principle can be illustrated by a simple leaf spring of the type used in light trucks and haul-it-yourself trailers. A spring, comprised of a single, thick, metal member with the strength necessary to support the load would result in too stiff a spring. But, when the strength is obtained by using a "stack" of separate, individually-flexing thin leaf elements, the spring rate is well within desired limits. Similarly, the spring rates of two-ply bellows proved to be significantly lower than those of single-ply bellows with twice the plate thickness.
Pressure and Seal Balance
Before you purchase your metal bellows seal, be sure to discern the proper pressure rating required by your application in relation to temperature, speed and sealed fluid lubricity. Determine if the seal is capable of handling reverse pressure, especially in dual pressurized seal arrangements.
Metal bellows seals are hydraulically balanced by locating the effective diameter of the bellows with respect to the seal face. Most standard bellows seals are typically balanced to approximately a 70/30 ratio. This means that 70 percent of the face contact area is above the effective diameter. More recently, 50 percent balanced bellows seals have been designed to handle both inner diameter (I.D.) and outer diameter (O.D.) pressure. Fifty percent balanced seals are able to handle reverse pressure, which is an upset condition that may occur in dual seal operation. Pressure-balanced by design, the welded metal bellows seal does not need a step in the shaft or sleeve required to balance the seal.
Additional Welded Metal Bellows Design Features:
- Plate Span: Narrower plate spans typically provide greater stability under pressure than wider spans.
- Face Width: Narrow face width typically results in less heat generation at the seal faces. Less heat improves face stability and narrow faces are less susceptible to coking than wider faces.
- Multitude of Metallurgies and Face Materials Available: Be sure your manufacturer offers an extensive array of materials such as AM-350, Hastelloy-C, Inconel 718, Alloy 20, Monel and Titanium.
- Design Characteristics: Consider not just the bellows seal head assemblies, but also the cartridge seal designs. Compare design features such as inside- or outside-mounted, reverse pressure capability, API 682 designs and high-temperature corrosion resistant designs.
Since metal bellows products vary greatly in what they are able to offer, it is important to understand the benefits and trade-offs as they apply to your particular operation. Be sure to review your applications and select the metal bellows seal best suited to help you extend meantime between repair, standardize inventory and increase reliability.